1. Field of the Invention
This invention relates generally to systems for supporting a tubular riser of a subsea well from a platform located on the ocean surface, and more particularly, but not by way of limitation, to such systems for supporting risers from a tension leg platform.
2. Description of the Prior Art
One problem encountered with offshore drilling and production platform is the provision of a suitable connection between the platform and a tubular riser connecting the subsea well to the platform, which connection must allow for relative movement between the platform and the riser.
Until very recently, substantially all offshore platforms have been rigid towers setting upon the ocean floor and extending upward above the surface of the ocean. With rigid towers the problem of relative movement between the riser and the surface platform at the top of the tower is generally less severe than that encountered with floating platforms, but some relative movement on the order of a foot or so may be encountered due to the thermal expansion of the riser when warm hydrocarbons are flowed therethrough.
Other floating structures of the prior art which have encountered more severe problems of relative movement between the surface platform and the tubular riser are typically on drilling vessels which float on the surface of the water, and which may have substantial movement relative to the riser. With these prior art drilling vessels, tensioning systems using hydraulic jacks have been utilized to support the riser while compensating for relative movement between the floating platform and the riser. An example of such a prior art device is shown, for example, in U.S. Pat. No. 3,981,357 to Walker et al., at FIG. 1 thereof.
One recent advance in the design of offshore drilling platforms is the vertically tethered floating platform generally referred to as a tension leg platform. A tension leg platform is a floating platform moored to the ocean floor by a plurality of vertical members which are placed under high tension loads due to excess buoyancy of the floating platform. Examples of such structures are shown in U.S. Pat. No. 3,648,638 to Blenkarn and U.S. Pat. No. 3,919,957 to Ray, et al.
The prior art additionally includes many systems using flexible elastomeric means for mounting one object upon another. Some of these are shown for example in:
U.S. Pat. No. 4,195,950 to Goldman PA0 U.S. Pat. No. 4,191,495 to Rivacoba, et al. PA0 U.S. Pat. No. 4,187,573 to Fyfe, et al. PA0 U.S. Pat. No. 4,152,087 to Zaleski-Zamenhoff et al. PA0 U.S. Pat. No. 4,134,702 to Thunes PA0 U.S. Pat. No. 3,730,463 to Richard PA0 U.S. Pat. No. 3,007,317 to Suderow, and PA0 U.S. Pat. No. 3,673,064 to Patterson, et al.
Elastomeric flex joints within risers are known, as shown for example in the description of such joints at page 7213 of the 1980-81 Composite Catalog of Oil Field Equipment and Services, illustrating a joint of Vetco Offshore Inc. designated by the trademark "Uni Flex".
Additionally, the prior art includes devices generally referred to as marine fenders which are elastomeric devices designed to absorb the kinetic energy of a berthing vessel to prevent damage to either the ship or the pier with which it is docking. These fenders typically provide a relatively constant force-displacement relationship. Examples of such fenders are shown in a catalog entitled "Lord Kinematics Marine Fenders" published by Lord Kinematics of Erie, Pennsylvania. Pages 9 and 10 of that publication, dated Mar. 15, 1976, illustrate a buckling column type marine fender. Pages 13 and 14 of that publication, dated Aug. 15, 1978 and Mar. 15, 1976, respectively, illustrate and describe a cylindrical column type marine fender.
The problems encountered in providing a connection between a tubular riser and a tension leg platform are different from those previously encountered with either rigid tower type platforms or with floating drilling ships, in that the relative movement between the tension leg platform and a riser connected thereto is sometimes greater than the movement experienced with rigid platforms but considerably less than the movement experienced with a floating drilling ship.
Also, with prior art rigid towers the well is often supported by a conductor casing which is an upward extension of the largest outer casing of the well borehole. In tension leg platforms, however it may be desirable to eliminate this outer casing to reduce the vertical loads on the platform, and then the smaller production risers must be supported directly from the platform.